This invention relates to thermal reactors used in semiconductor wafer processing operations and, more particularly to a system for loading and unloading a wafer.
Semiconductor substrate, or wafers, are typically processed by chemical vapor deposition. Components included in semiconductor processing operations include a reaction chamber which is heated to a desired temperature and is configured to facilitate the controlled flow of a reactant gas, which contains the material to be deposited by thermal reaction onto a wafer. A base, which is commonly referred to in the art as a "susceptor," is usually provided in the reaction chamber for supporting the wafer during chemical vapor deposition. To facilitate automated processing, a robotic arm has been employed to place a wafer on a susceptor and subsequently, after processing, to remove it from the reactor.
Susceptors have evolved considerably during the last ten years, from simple flat platforms, which contributed nothing beyond their physical support to the processing operation, to susceptors provided with mechanisms for rotating the wafer during processing and sophisticated systems for sensing and responding to local temperature differences at the susceptor surface. Further, means for displacing the wafer from the susceptor after processing have been provided to assist in removal of the wafer by the robotic arm. Such innovations in susceptor design have contributed significantly to improved semiconductor quality and uniformity.
The means for facilitating wafer displacement from the susceptor surface and transfer to the robotic arm for removal from the reactor, remains problematic in automated semiconductor processing systems. One approach to wafer transfer operations, known in the art, involves using wafer support pins, which move vertically through holes in the susceptor to effect displacement of the wafer from the susceptor surface after processing.
The use of wafer support pins has several important shortcomings. Abrasion of the pins within the holes in the susceptor is caused both by the rotation of the susceptor as well as the different rates of thermal expansion exhibited by the stainless steel or quartz pins and the graphite susceptor. The abrasion results in particles which can contaminate the processing environment and compromise the quality of the processed semiconductor wafer. In addition to abrasion within the holes, marring of the backside of wafers has also been observed due to shrinkage of the hot wafers while supported on the relatively sharp pins. The longer cool down periods required to prevent such wafer damage tends to reduce reactor throughput.
A further problem associated with the use of wafer support pins is the permeation of processing gases through spaces that necessarily exist between the pins and the holes in the susceptor. As a result, the processing gases may deposit on the backsides of the wafers. Moreover, the presence of holes in the susceptor results in temperature non-uniformity both in the susceptor and in the wafer being supported thereon.
One approach taken to improve the pin-based wafer transfer operation is disclosed by U.S. Pat. No. 5,421,893 to Perlov. In that invention, the wafer displacement mechanism employs wafer support pins which are suspended from the susceptor. The use of freely suspended pins, as opposed to earlier pins, which were connected with both the vertical and rotational drive mechanisms, is intended to reduce pin abrasion within the holes in the susceptor during rotation. Further, the Perlov support pins have enlarged frusto-conical heads which fit into complementary depressions countersunk in the upper surface of the susceptor, providing a flat support surface and a sealing means for decreasing the permeation of processing gases to wafer backsides.
Notwithstanding the improvements related to diminished rotational abrasion and backside deposition, Perlov still employs a susceptor with a plurality of holes, which are likely, in view of the prior art, to cause temperature non-uniformities. In addition, contaminating particles are still likely to be generated by abrasion when the support pins slide vertically within the susceptor holes. Finally, shrinking of the hot wafer on support pins may still result in backside scratching. Thus, Perlov fails to resolve several of the most important problems inherent in the use of support pins as a means of separating the processed wafer from the susceptor.
Another approach to wafer transport is disclosed in U.S. Pat. No. 5,080,549 to Goodwin, et al. That invention relates to a wafer handling system which utilizes the Bernoulli Principle to effect contactless pick up of the wafer. Specifically, a robotic arm is adapted to include a plurality of gas outlets in the bottom plate of a pick up wand. The gas outlets radiate outward from a central portion of the wand in such a pattern as to produce an outward flow of gas across the top surface of the wafer. The gas flow creates an area of relatively low pressure between the top surface of the wafer and the bottom surface of the pick up wand, resulting in wafer pick up without physical contact. Unfortunately, while the Bernoulli wand addresses all of the major shortcomings associated with pin-based wafer transfer mechanisms and has some other advantages, it presents a different problem. In order to provide adequate gas flow, the robot arm and pick up wand assembly is too thick to fit between the unprocessed wafers in a standard wafer supply cassette and can stir up particles.
Therefore, the need exists for a mechanism which facilitates wafer loading from standard cassettes as well as unloading from a susceptor without using wafer support pins, thereby reducing particle contamination within the reactor, temperature non-uniformities, and backside damage during hot pick up.